This invention relates to a method for generating a duobinary signal and an optical transmitter using the same method.
Recently, an optical duobinary technique has attracted attention as an optical transmission manner which can overcome the waveform deterioration due to a chromatic dispersion. The duobinary technique itself has been researched for a long time and its theory system was established in the time of pulse communication with a coaxial cable. The duobinary technique is that a signal bandwidth(spectrum width) is reduced to less than xc2xd by mapping a binary data signal to be transmitted into a three-level signal with a redundancy in the amplitude direction. It has a merit that the waveform deterioration due to a dispersion etc. is difficult to happen since the spectrum width is narrowed. However, it had never attracted attention in high-speed optical communication since, in the receiver, a receiving circuit with a linearity is required to handle the three-level signal and a decoder for decoding the original binary data signal from the three-level signal is necessary.
A. J. Price et al., xe2x80x9c210 km Repeaterless 10 Gb/s Transmission Experiment Through Nondispersion-Shifted Fiber Using Partial Response Schemexe2x80x9d, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol.7, No.10, pp.1219-1221(1995) reports an optical duobinary technique where a redundancy is given to optical phase.
The optical transmitter used in this optical duobinary technique is shown in FIG. 1. A binary data signal is passed through a low-pass filter, which is ideally a cosine roll-off filter, with a bandwidth of about 0.25 times a clock frequency. Due to the excessive limitation of bandwidth, the interference between codes is occurred to convert the binary data signal into a three-level data signal. Similarly, a binary inverted data signal is converted into a three-level data signal. Then, these signals are input with an amplitude equal to a half-wavelength voltage Vxcfx80 to a push-pull optical intensity modulator. The push-pull optical intensity modulator is a Mach-Zehnder(MZ) interferometer with modulation terminals connected to both arms, where unnecessary chirp(phase variation) does not occur. In this technique, the bias voltage is so adjusted that a three-level signal(xe2x88x921, 0, 1) corresponds to a mountain(ON), a valley(OFF) and a neighboring mountain(ON) in the voltage-extinction characteristic of the push-pull optical intensity modulator. As a result, when the amplitude and phase of light are represented by (A, "PHgr"), the data signal is mapped into three states of (1, 0), (0, indefinite) and (1, xcfx80) to generate optical duobinary signal light. This three-level signal light can be, as it is, decoded into the binary signal composed of 1 and 0 since the phase information is deleted by square-law detection when the direct detection is conducted by an optical detector. This means that direct-detection optical receivers, which are widely used, can be used as it is. It is one of the reasons why the duobinary technique has attracted attention again.
Japanese patent application laid-open No.8-139681(1996) discloses another optical duobinary system as shown in FIG. 2. In this system, as shown in FIG. 2, a binary transmission data signal 50 is converted into a three-level duobinary signal by a code converter 51. In the code converter 51, the code conversion is first conducted by a precoder 52 composed of an exclusive-OR circuit 26 and an 1-bit delay circuit 27, and then the duobinary signal is generated by a binary-to-three-level converter 53 composed of an 1-bit delay circuit 27 and an adder 54. The duobinary signal is divided into two signals, where the first signal divided is input through an amplitude adjusting circuit 55 and a bias adjusting circuit 56 to the first input terminal of an optical modulator 58 and the second signal divided is input through an inverter 57 and an amplitude adjusting circuit 55 to the second input terminal of the optical modulator 58. The optical modulator 58 is a Mach-Zehnder optical intensity modulator, where light from a light source 1 is modulated by applying the first and second signals to its two optical waveguides to generate the optical duobinary signal.
When the two electrical signals with an amplitude equal to a half-wavelength voltage(vxcfx80) of the optical modulator 58 are input and the bias point of signal is set at a point (a) of transmission characteristics 59 of the modulator as shown in FIG. 3, the middle value of the duobinary signal 60 is assigned to a minimum transmittance state and the minimum and maximum values thereof are assigned to maximum transmittance states, where the optical phase is inverted by 180 degree between the minimum and maximum values. As a result, the three levels of the electrical signal can be assigned to the optical three states, thereby narrowing the modulated light spectrum. Meanwhile, this system has a composition equivalent to the system in FIG. 1 where the low-pass filters are replaced by the binary-to-three-level converter 53.
However, in the conventional methods, the driving amplifier of the modulator requires a linearity since the electrical signal for driving the optical modulator is three-level. On the other hand, the driving amplifier generally needs a high-output characteristic greater than 5 Vp-p. Therefore, there is a problem that designing the circuit becomes very difficult since the linearity and the high-output characteristic are required therein.
Accordingly, it is an object of the invention to provide a method for generating a duobinary signal where an electrical signal for driving an optical modulator is binary.
It is a further object of the invention to provide an optical transmitter where a binary electrical signal for driving an optical modulator is used to generate a duobinary signal.
According to the invention, a method for generating a duobinary signal, comprises the step of:
modulating individually an intensity and a phase of a carrier wave.
According to another aspect of the invention, a duobinary-manner optical transmitter, comprises:
a laser device which outputs signal light;
an optical intensity modulator which intensity-modulates the signal light according to a first data signal generated by dividing a data signal into two signals;
a precoder which inputs a second data signal generated by dividing the data signal into two signals; and
an optical phase modulator which phase-modulates the intensity-modulated signal light according to a signal which is obtained delaying 0.5 bit an output signal of the precoder.
According to another aspect of the invention, a duobinary-manner optical transmitter, comprises:
a precoder which inputs a second data signal generated by dividing a data signal into two signals;
a direct modulation phase shift keying encoder which inputs an output of the precoder after delaying 0.5 bit the output;
a laser device which outputs signal light phase-modulated by modulating an injected current according to an output of the direct modulation phase shift keying encoder; and
an optical intensity modulator which intensity-modulates the phase-modulated signal light according to a first data signal generated by dividing the data signal into two signals.
According to another aspect of the invention, a method for generating a duobinary signal, comprises the steps of:
providing two carrier waves with an equal frequency;
intensity-modulating individually the two carrier waves by first and second intensity modulators; and
coupling the two intensity-modulated carrier waves so that they have a phase difference of xcfx80.
According to another aspect of the invention, a duobinary-manner optical transmitter, comprising:
a laser device which outputs signal light;
an optical divider which divides the signal light into two light signals;
a first optical intensity modulator which inputs first signal light divided by the optical divider;
a second optical intensity modulator which inputs second signal light divided by the optical divider;
an optical coupler which couples output lights of the first and second optical intensity modulators after phase-shifting at least one of the output lights so as to give a phase difference of xcfx80 between the output lights of the first and second optical intensity modulators; and
a precoder which inputs a data signal;
wherein the first optical intensity modulator is driven by a first encoded signal generated by dividing an encoded signal to be output from the precoder into two signals, and the second optical intensity modulator is driven by a signal which is obtained by delaying 1 bit a second encoded signal generated by dividing the encoded signal into the two signals, thereafter inverting 0 and 1 each other.
According to another aspect of the invention, a duobinary-manner optical transmitter, comprises:
a laser device which outputs signal light;
an optical intensity modulator which intensity-modulates the signal light according to a first data signal generated by dividing a data signal into two signals;
a precoder which inputs a second data signal generated by dividing the data signal into two signals; and
an optical phase modulator which phase-modulates the intensity-modulated signal light according to a signal which is obtained delaying 0.5 bit an output signal of the precoder;
wherein a waveform of the signal light is varied by changing an operating point of the optical intensity modulator.
According to another aspect of the invention, a duobinary-manner optical transmitter, comprises:
a laser device which outputs signal light;
an optical intensity modulator which intensity-modulates the signal light according to a first data signal generated by dividing a data signal into two signals;
a precoder which inputs a second data signal generated by dividing the data signal into two signals; and
an optical phase modulator which phase-modulates the intensity-modulated signal light according to a signal which is obtained delaying 0.5 bit an output signal of the precoder;
wherein a waveform of the first data signal is varied through a non-linear electric circuit.
According to another aspect of the invention, a method for generating a duobinary signal, comprises the step of:
modulating individually an intensity and a polarization of a carrier wave.
According to another aspect of the invention, a duobinary-manner optical transmitter, comprises:
a laser device which outputs signal light;
an optical intensity modulator which intensity-modulates the signal light according to a first data signal generated by dividing a data signal into two signals;
a precoder which inputs a second data signal generated by dividing the data signal into two signals; and
an optical polarization modulator which polarization-modulates the intensity-modulated signal light according to a signal which is obtained delaying 0.5 bit an output signal of the precoder.
In the invention, an intensity modulator and a phase modulator are cascade-connected, and the amplitude(or intensity) and phase of signal light are individually modulated. Meanwhile, an intensity modulation signal and a phase modulation signal are input to the intensity modulator and phase modulator, respectively, while having predetermined conversion and phase relations. The conversion and phase relations will be explained below. FIG. 4 shows calculation results of the amplitude and phase of optical duobinary signal light modulated by a conventional three-level signal. As shown in FIG. 4, in the optical duobinary signal light, the phase is inverted from 0 to xcfx80 or from xcfx80 to 0 at a point where the amplitude is 0. The phase inversion occurs at the middle point of a 1 time slot. The chase does not change when the amplitude is 1. The characteristic that xe2x80x9cthe phase is inverted at a point where the amplitude is 0xe2x80x9d gives the characteristics of optical duobinary manner that have a narrowed optical spectrum and a high durability against dispersion.
When a data signal is transmitted carrying on an optical amplitude(or intensity) and is directly detected by an optical receiver to get the data signal, only the phase modulation signal needs to be encoded by a precoder(encoder). The rule of the encoding is, as described earlier, that xe2x80x9cthe phase modulation signal is inverted when the intensity modulation signal is 0xe2x80x9d. This can be easily achieved by EX-NOR(inverted output of exclusive xe2x80x9corxe2x80x9d) and a 1-bit delay circuit, as shown in FIG. 9. The precoder uses an output value 1 bit before, therefore the output is inverted depending on an initial output value. However, there is no problem because an absolute optical phase has no meaning for optical duobinary signal light. After delaying 0.5 bit the phase modulation signal to the intensity modulation signal, the intensity-modulated signal light is phase-modulated (0xe2x88x92xcfx80). Thereby, the optical duobinary signal light can be generated. In such cascade modulation, any one of the optical phase modulator and the optical intensity modulator may be placed at the first.
Also, two lights with an equal frequency may be provided by, for example, dividing signal light into two lights, then turning OFF(or ON) either of the two lights or both of them by two optical intensity modulators, thereafter coupling them to give a phase difference of u between the outputs from the two optical intensity modulators. As a result, the data signal can be mapped into three optical states as described earlier to generate optical duobinary signal light(parallel type). In this case, a parallel-type precoder is necessary for the input data signal in addition to the above-mentioned precoder. The parallel-type precoder can be, as shown in FIGS. 20 and 21, composed of a simple circuit using a 1-bit delay circuit and an inverter.
Also, in the invention, by properly setting the optical intensity modulation waveform, modulated light closer to that in the optical duobinary modulation manner using the three-level signal can be obtained. In an ideal optical duobinary waveform, the cross point is biased upward as shown in FIG. 5A, where the optical spectrum has very small high-frequency components as shown in FIG. 5B. To suppress the high-frequency components, the invention approaches the ideal optical duobinary by using a non-linear modulation characteristic of the optical intensity modulator or an electric circuit with a non-linear input characteristic. When a signal 62 is input as shown in FIG. 6A, the obtained modulation light spectrum has high-frequency components to be remained as shown in FIG. 7A. On the contrary, when a signal 64 is input shifting the bias value, the output optical waveform 65, which is biased to the side of optical transmission, becomes closer to the ideal duobinary waveform, and the high-frequency components can also be suppressed as shown in FIG. 7B.
On the other hand, in the composition of cascade-connected intensity modulator and phase modulator, when the main axis direction of polarization of output signal light is supplied to be 45xc2x0 to the optical axis of the phase modulator, only the signal light component in the optical axis direction is phase-modulated. Therefore the polarization of signal light can be modulated according to phase modulation. As a result, the spectrum of the signal light is not so narrowed, but the polarization-modulated duobinary signal can be generated.